In recent years, attention has been paid to titanium oxide particles in the field of photocatalysis. For example, Patent Literatures 1 and 2 and Non-Patent Literatures 1 to 3 disclose titanium oxide particles which have a decahedral box-like shape and are mainly formed of anatase-type crystals (hereinafter, referred to as “decahedral titanium oxide particles”).
Patent Literatures 1 and 2, and Non-Patent Literatures 1 and 3 describe that since decahedral titanium oxide particles have large surface areas per unit mass and have high crystallinity with fewer internal defects, the titanium oxide particles have high activity as photocatalysts. Furthermore, Non-Patent Literature 2 describes that decahedral titanium oxide particles have a high ratio of the highly reactive (001) plane and are promising as photocatalysts.
With regard to a method for producing decahedral titanium oxide particles, there is available, for example, a method of using a hydrothermal reaction using hydrofluoric acid described in Non-Patent Literature 2. However, this production method involves a constitution of using hydrofluoric acid, and is therefore not suited for production on an industrial scale.
The methods for producing decahedral titanium oxide particles described in Patent Literatures 1 and 2, and Non-Patent Literatures 1 and 3, are methods for producing titanium oxide particles (TiO2) by introducing titanium tetrachloride (TiCl4) vapor and oxygen (O2) gas into a reaction tube, subsequently heating these gases from the outside of the reaction tube, and thereby inducing a reaction represented by the following reaction formula (1):TiCl4+O2→TiO2+2Cl2  (1)
When the production methods described above are used, a powder product containing titanium oxide particles can be obtained in the downstream side of the reaction tube. This powder product contains a large proportion of decahedral titanium oxide particles.
However, the production methods described in Patent Literatures 1 and 2 and Non-Patent Literatures 1 and 3 employ systems of heating titanium tetrachloride and oxygen gas, which are raw materials, from the outside of the reaction tube when the temperature of the raw material gas is to be rapidly raised to the temperature at which the thermal oxidation reaction of titanium tetrachloride proceeds. Therefore, when a raw material gas containing titanium tetrachloride and an oxidizing gas is used at a high flow rate, a thermal conductivity from the heat source at the outside of the reaction tube to the raw material gas inside the reaction tube is not secured to the extent required for complete consumption of titanium tetrachloride in the raw material gas through the thermal oxidation reaction, the reaction is not completed, and unreacted titanium tetrachloride remains behind in an area downstream of the reaction zone. As a result, there occurs a problem with a decrease in the yield of the powder product that is obtained.    [Patent Literature 1] Japanese Patent No. 4145923    [Patent Literature 2] Japanese Patent Application Laid-Open No. 2006-52099    [Non-Patent Literature 1] KUSANO Daisuke, TERADA Yoshihiro, ABE Ryu, and OHTANI Fumiaki, 98th Shokubai Toronkai (Catalyst Forum) (September, 2006), Proceedings of Forum A, p. 234    [Non-Patent Literature 2] Hua Gui Yang, et al., Nature, Vol. 453, p. 638-p. 641    [Non-Patent Literature 3] Amano F., et al., Chem. Mater., 21, 2601-2603 (2009)